Serotonin (5-hydroxytryptamine, 5-HT), one of the neurotransmitters, is broadly distributed throughout human body including both the central nervous system and the peripheral nervous system. Approximately 95% of the human body's total serotonin is found in the gastrointestinal tract, while about 5% thereof is found in the brain. Serotonin receptors are located in intestinal nerves, enterochromaffin cells, intestinal smooth muscle, immune tissues, etc. Serotonin receptor subtypes include 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6, and 5-HT7. Interactions between these various receptors and serotonin are linked to various physiological functions. Therefore, various researches have been performed for developing therapeutic agents that are capable of interacting with a specific serotonin subtype as a target. The researches include identification of 5-HT4 receptors and active agents interacting therewith (Langlois and Fischmeister, J. Med. Chem. 2003, 46, 319-344).
It has been found by the previous literatures that 5-HT4 receptor agonists are useful for treating an abnormal gastrointestinal motility, i.e., dysfunction in gastrointestinal motility. The abnormal gastrointestinal motility may result in various disorders, for example irritable bowel syndrome (IBS), constipation, dyspepsia, delayed gastric emptying, gastroesophageal reflux disease (GERD), gastroparesis, post-operative ileus, intestinal pseudo-obstruction, drug-induced delayed transit, etc.
Representative 5-HT4 receptor agonists disclosed in prior arts include tegaserod (an aminoguanidine derivative, U.S. Pat. No. 5,510,353), prucalopride (a benzofuran carboxamide derivative, EPO445862), cisapride (a benzamide derivative, U.S. Pat. No. 4,962,115), mosapride (EP0243959), etc. These compounds are known as an agent stimulating gastrointestinal motility.